High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319, and in Pengilly, U.S. Pat. No. 3,047,539, incorporated herein by reference. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection-moldable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction. Recently, block copolyesters, wherein the major portion of the repeating units are poly(1,4-butylene terephthalate) blocks, have been found to have enhanced impact resistance. See Borman et al., U.S. Pat. Application Ser. No. 752,325, filed on Dec. 20, 1976. Brinkman et al., U.S. Pat. No. 3,580,965 disclose that the impact strength of linear high molecular weight polyesters can be improved by adding .alpha.-olefin-vinyl ester copolymers. Seydl et al., U.S. Pat. No. 3,937,757 disclose that poly(1,4-butylene terephthalate) can have its electrical arc tracking resistance improved by adding polyolefins, among which is suggested a copolymer of ethylene and vinyl acetate. Holub et al. U.S. Pat. No. 4,122,061 describe glass-reinforced combinations of poly(1,4-butylene terephthalate) and poly(ethylene terephthalate) to which olefin based copolymer resins can be added in surprisingly small amounts to provide improved impact strengths. Cohen et al., U.S. Ser. No. 866,059, filed Dec. 30, 1977 and Cohen, U.S. Ser. No. 866,008, filed Dec. 30, 1977, disclose, respectively, the use of small amounts of ethylene-vinyl acetate copolymer to improve the impact strength of unreinforced, unfilled poly(1,4-butylene terephthalate) and to improve the mold release of talc-filled poly(1,4-butylene terephthalate) or a block copolyester thereof with aliphatic/aromatic polyesters. The disclosures of the above-mentioned patents and applications are incorporated herein by reference.
It has now been discovered that if a small amount of a olefin-based copolymer is incorporated in compositions comprising mineral-filled polyester resin combinations or mineral-filled and/or reinforced block copolyesters, the compositions exhibit vastly improved impact strengths. Also the deflection temperature under load (DTUL) is reduced by less than the expected amount, and the modulus is decreased only slightly. Such results permit the achievement of physical properties heretofore obtained only by post-reaction (solid state polymerization) of prior art compositions.